The objectives of this proposal are to characterize lymphocyte subpopulations and the mechanisms by which they regulate in vitro erythropoiesis. Specifically we plan to develop in vitro tests which can be shown, by in vivo correlates, to be useful for identifying mechanisms of aplastic anemia and possibly predicting the outcome to various therapeutic protocols used for treating this disease. The in vitro assays would involve growing immature erythroid precursors, known as erythroid burst forming units (BFU-E) and determining to what extent that growth is affected by various subpopulations of normal and abnormal lymphocytes. The lymphocyte subpopulations to be evaluated would be characterized by a panel of monoclonal antibodies which would be used in cytotoxic assays to negatively select cell populations, or in combination with the fluorescence activated cell sorter (FACS II) to positively select cell populations. The FACS II will also be used to study the kinetics of transfusion induced sensitization in dogs and determine the effectiveness of immunosuppressive protocols in preventing sensitization and subsequent marrow graft rejection.